Musician with it groans terribly (8)
I believe the answer is:
organist
'musician' is the definition.
(I know that organist is a type of musician)
'with it groans terribly' is the wordplay.
'with' says to put letters next to each other.
'terribly' is an anagram indicator.
'it' next to 'groans' is 'itgroans'.
'itgroans' with letters rearranged gives 'ORGANIST'.
(Other definitions for organist that I've seen before include "It groans for church musician" , "Church instr. player" , "Cathedral musician" , "Accompanist, keyboard player" , "One plays in church usually" .)